It is usual to produce evaporators or heat exchangers, exchanging heat between a cooling fluid and air, as a stack of plates, virtually identical for preference, positioned symmetrically in pairs. A pair of plates arranged opposite each other forms a substantially U-shaped duct whose inlet is connected to the outlet of the U-shaped duct of the preceding pair of plates (if there are any, that is to say if it is not the first pair of plates), and whose outlet is connected to the inlet of the U-shaped duct formed by the next pair of plates (if there are any, that is if it is not the last pair of plates). Cooling air flows between the consecutive pairs of plates. It is known for stamped dimples to be formed on at least one of the pairs of plates forming a duct. These dimples are identical in shape, position and orientation. They project into the interior of the duct formed by the pairs of plates and thus allow better heat exchange by agitating the cooling fluid flow, and especially by promoting its movement in a turbulent flow. These dimples can be formed by an assembly method, particularly by brazing two bosses opposite each other. In this case, the plates forming a pair of plates are the same as one another, and each boss has an equivalent height of half the depth of the U-shaped duct, that is to say of the distance from the opposing plates.
Moreover, the dimples can be formed by brazing the bosses of a first plate against the flat surface of a second plate. In this case, the two plates of a pair of plates can be identical and each dimple has a height equal to the depth of the U-shaped ducts.
Unfortunately the flow of cooling fluid in this known type of evaporator produces a noise, particularly a whistling, which is extremely unpleasant for the vehicle occupants, and vibrations detrimental to the longevity of the evaporator. The amplitude of the radiated noise is accentuated by the geometry of the evaporator including an overlaying of the sets of plates.